Pyco2sys

**PyCO2SYS** is a Python implementation of CO₂SYS, based on the [MATLAB v2.0.5](https://github.com/jamesorr/CO2SYS-MATLAB) but also including the updates made for tentatively forthcoming MATLAB v1.21 as well as some additional related calculations. This software calculates the full marine carbonate system from values of any two of its variables.

*Changes in v1.2.1:*

* Fixed typo in [ESM10](https://pyco2sys.readthedocs.io/en/latest/refs/#ESM10) equations that had been carried through into `extra.buffers_ESM10` function (thanks [Jim Orr](https://twitter.com/James1Orr/status/1248216403355803648)!).

* Added module `extra` containing functions to calculate variables not included in CO2SYS for MATLAB:
* `buffers_ESM10` calculates the buffer factors of [ESM10](../refs/ESM10), corrected for the typos noted by [RAH18](../refs/RAH18).
* `bgc_isocap` calculates the "exact" isocapnic quotient of [HDW18](../refs/HDW18), Eq. 8.
* `bgc_isocap_approx` calculates the approximate isocapnic quotient of [HDW18](../refs/HDW18), Eq. 7.
* `psi` calculates the $\psi$ factor of [FCG94](../refs/FCG94).
* Added all functions in `extra` to the `CO2dict` output of the main `CO2SYS` function, and documented in the [Github repo README](https://github.com/mvdh7/PyCO2SYS#pyco2sys).

* Removed unnecessary `WhoseTB` input to `assemble.equilibria`.

Adding extra optional inputs for consistency with Pierrot et al.'s tentatively forthcoming MATLAB "v1.21". Continuing to reorganise subfunctions into more Pythonic modules, while avoiding changing the actual mechanics of calculations.

* Updated pH-solving iterative functions so that iteration stops separately for each row once it reaches the tolerance threshold.
* Extracted all functions for solving the CO₂ system into a separate module (`solve`).
* Extracted other key subfunctions into module `assemble`.
* Added total ammonium (`NH3`) and hydrogen sulfide (`H2S`) concentrations as optional inputs to be included in the alkalinity model.
* Added optional input to choose between different equations for hydrogen fluoride dissociation constant (`KFCONSTANT`).
* Added functions to enable carbonate ion as an input carbonate system variable.
* Output is now only the `CO2dict` dict, not the original `DATA`, `HEADERS` and `NICEHEADERS`.
* Eliminated all global variables throughout the entire program.

* Extracted all equations for concentrations and equilibrium constants into functions in separate modules.

* Eliminated all global variables from the `_Constants` function.

* Moved the as-close-as-possible version into module `original`. The default `from PyCO2SYS import CO2SYS` now imports the more Pythonic implementation.